1. Field of the Invention
The present invention relates to signal acquisition and processing. In particular, the present invention relates to a system that combines Adaptive Antenna Arraying and Adaptive Locally-Optimum Detection signal processing techniques and achieves the capabilities of both techniques. The system automatically distributes interference signals among Adaptive Antenna Arraying and Adaptive Locally-Optimum Detection sub-systems according to which sub-system""s algorithms can provide a more optimum suppression of the interference signals. The present invention also relates to a system that combines Adaptive Filtering and Adaptive Locally-Optimum Detection signal processing techniques and achieves the capabilities of both techniques. The present invention also relates to a system that combines Adaptive Filtering, Adaptive Antenna Arraying and Adaptive Locally-Optimum Detection signal processing techniques and achieves the capabilities of all three techniques. The system is primarily designed for incorporation within systems using Direct Sequence Spread Spectrum signals, such as Global Positioning System (GPS) signals. In a preferred embodiment the inventive system is incorporated within a conventional GPS receiver, which tends to be vulnerable to jamming by interference signals.
2. Description of the Related Art
Adaptive Filtering, Adaptive Antenna Arraying and Adaptive Locally-Optimum Detection are three complementary signal processing techniques used by an Adaptive Filter (AF) system, an Adaptive Antenna Array (AAA) system and an Adaptive Locally-Optimum Detector (ALOD) system, respectively, for detecting weak signals (such as radio and radar) in the presence of strong man-made interference or jamming signals.
Each one of these techniques has its own limitations. An AF, such as Frequency Excision (FX) or Adaptive Wiener Filtering (AWF), can be effective against a large number of narrowband jammers, but its performance declines with interference bandwidth. An AF is ineffective against full-band jammers. An AAA, such as a Controlled Radiation Pattern Antenna (CRPA), is effective against all types of jamming, but requires an array of multiple antenna elements. An ALOD, such as Nonlinear Adaptive Processor (NONAP) technology developed by The Johns Hopkins University/Applied Physics Lab (JHU/APL), achieves gain against all power-efficient (and therefore non-Gaussian) types of jamming and requires relatively little processing power, but produces little gain when confronted with nearly Gaussian interference, such as more than two or three simultaneous independent jammers of comparable power.
The AAA system suppresses interference signals based on their direction of arrival or polarization. A conventional prior-art AAA system accomplishes this by weighting the signal from each antenna element and summing them together to form an output. For applications where the signal of interest is weaker than the antenna noise so that any detectable signal above the antenna noise is interference and should be suppressed, the output may be used as an error feedback signal to drive the AAA weights towards the values that minimize the output and therefore suppress the interference contribution to the output. Such a prior-art AAA system configuration is shown by FIG. 1.
For appropriate applications, a conventional prior-art AAA system can incorporate xe2x80x9cbeam-formingxe2x80x9d capability, i.e. the ability to protect a known direction of arrival from which the signal of interest is expected so that the AAA will not form a null in that direction. For example, a conventional Griffiths-Jim beamforming pre-processor (as described in Griffiths, L. J., and Jim, C. W., xe2x80x9cAn Alternative Approach to Linearly Constrained Adaptive Beamforming,xe2x80x9d IEEE Trans. Antennas Propag., vol. AP-30, pp. 27-34, (January 1982)) can simply be inserted just in front of the AAA system as shown by FIG. 2. The Griffiths Jim pre-processor first weights the signal from each of the M elements, X, so that if summed together without further weighting they would steer the array in the direction of the desired signal. The Griffiths-Jim reference output component is taken to be that unweighted sum of the steered input components, and the M-1 Griffiths-Jim auxiliary components are simply the differences between independent pairs of the steered elements of X.
Since a standard AAA system can only suppress M-1 spatially distinct interferers, where M is the number of antenna elements, prior art includes augmenting an AAA system with an AF system. Adding the AF component increases the number of simultaneous sources of interference that may be countered if some of these sources are narrow-band.
An AF system can improve the performance of an AAA system in another way: If, as is usual, the AAA weights are simple multipliers that shift signal phase but don""t alter the time delay, then the ability of the AAA system to cancel interference declines as interference bandwidth increases, or if signal echoes, such as may be caused by multi-path propagation, are present. Under such conditions, an AF system can improve antijam (AJ) gain by altering the effective time delay of signals from a given element, or even by suppressing echoes.
A conventional prior-art means of combining AF and AAA techniques is to filter the M signals from the antenna elements with M separate filters prior to applying these signals to the AAA system. As with the AAA system, the output of the AAA system is used as an error feedback signal, but in the AF-AAA system this error signal is used to update not only the AAA weights, but also a set of filter weights (one for each filter xe2x80x9ctapxe2x80x9d or degree of freedom) for each filter. Such a prior-art AF-AAA system, sometimes referred to as a Space-Time Adaptive Processor (STAP), is shown by FIG. 3.
An amplitude-domain ALOD, such as described in (1) Higbie, J. H., xe2x80x9cAdaptive Nonlinear Suppression of Interference,xe2x80x9d in Proc. MILCOM 88, IEEE, New York, p. 23.3.1 (1988); (2) Higbie, J. H., xe2x80x9cAdaptive Locally-Optimum Detection Signal Processor and Processing Methods,xe2x80x9d U.S. Pat. No. 5,018,088, issued May 21, 1991; and (3) Radcliffe, S. T., and Higbie, J. H., xe2x80x9cEvaluating Transform Estimators for Locally Optimum Signal Processors,xe2x80x9d Unclassified paper in Classified Proc. MILCOM 92, IEEE, New York, p. 162 (1993), enables suppression of non-Gaussian interference sources (i.e. ones whose amplitudes do not follow a Rayleigh probability distribution). This class includes all man-made interference sources, such as jamming. An ALOD is especially attractive in an AJ system because it can suppress wide-band jamming, unlike an AF, and doesn""t require multiple antenna elements, as does an AAA. Another advantage of an ALOD implemented as described in References 1-3 is that its adaptation is rapid, so that it can combat even rapidly time-varying jamming, which is difficult for conventional AAA or AF signal processing designs that are based on using error feedback to update weights.
From the perspective of the jammer, it is possible to design jamming waveforms against which an ALOD system does not provide usable antijam (AJ) gain, however transmitting such waveforms nearly always requires a reduction in the power that can be devoted to jamming the victim receiver. The jamming power must be reduced either by spreading its energy outside the frequency band occupied by the victim signal, or by modulating the amplitude of the jamming waveform. (Amplitude modulation reduces the average jamming power relative to the peak jamming power. Since the jammer""s peak power is typically fixed, amplitude modulation causes an absolute reduction in the average jamming power, which reduces jamming impact.)
It is advantageous to augment an ALOD system with an AF system, in order to increase the number of simultaneous independent jammers that may be countered. An effective prior-art AF-ALOD system configuration is to feed the (single-element) input signal to the AF system and to feed the output from the AF system to the ALOD system. No feedback path is required.
As mentioned in (4) Brunson, S. J., xe2x80x9cCombined Narrowband Frequency Excision and Nonlinear Zero-Memory Processor,xe2x80x9d Unclassified paper in Classified Proc. MILCOM 92, IEEE, New York, p. 156 (1993), when a wide-band jamming signal, such as may effectively be suppressed by an ALOD system, is filtered, amplitude fluctuations are introduced that reduce the antijam (AJ) gain achievable by the ALOD system. An effective integration of AF and ALOD techniques therefore requires that the AF system be designed to minimize the reduction in ALOD AJ gain.
As also mentioned in Reference 4, an effective prior art-approach for designing an AF system as a pre-processor for an ALOD system is to restrict filtering so that the AF system only removes very narrow frequency slices when wideband jamming is present, and always preserves the phase relationships of the various frequencies. An AF system operating in the frequency domain, typically called a Frequency Excision (FX) system, can do this if appropriately designed. However, conventional prior-art AF systems have been designed to operate alone or as AAA pre-processors and do not perform effectively as ALOD or AAA-ALOD pre-processors. In particular, they do not suppress amplitude fluctuations in the AF output that goes to the ALOD, and can even make such amplitude fluctuations larger, thereby preventing the ALOD from achieving significant AJ gain.
Attempts have been made in the past to combine the advantages of both AAA and ALOD systems. For example, H. Schmidt and J. W. Bond investigated a two-element electrically-small antenna array followed by a separate locally-optimum detector (See IEEE MILCOM94 Proceedings, page 11-2.1 to 3, October 1994). However, Schmidt and Bond were unable to solve the general adaptation problem, i.e., to have their system adapt quickly to a rapidly changing interference scenario, except through a brute force approach that required excessive computation; Schmidt and Bond designed a sub-optimum approach for determining weights of the antenna array which would produce the desired result under certain restricted interference scenarios.
A need therefore exists for a system that solves the general adaptation problem and is able to integrate or combine the jamming suppression capabilities of signal processing techniques associated with AAA and ALOD systems according to the characteristics of the incoming interference signals. A need also exists for a system that is able to effectively combine the jamming suppression capabilities of AF signal processing techniques with those of ALOD systems, according to the characteristics of the incoming interference signals. A need also exists for a system that is able to combine the jamming suppression capabilities of AF signal processing techniques with those of AAA and ALOD systems, according to the characteristics of the incoming interference signals.
Further, a need also exists for a system that is computationally simple; converges towards an optimum solution for arbitrary interference scenarios; and can be used with an antenna array having an arbitrary number of antenna elements.
The present invention provides a system that combines the advantages of Adaptive Antenna Arraying and Adaptive Locally-Optimum Detection, as well as Adaptive Filtering, signal processing techniques and achieves the capabilities of all these techniques. The present invention represents the first successful integrated system for performing signal processing techniques associated with an Adaptive Antenna Array (AAA) system and an Adaptive Locally-Optimum Detection (ALOD) system. The system automatically distributes interference signals among Adaptive Filtering, Adaptive Antenna Arraying and Adaptive Locally-Optimum Detection sub-systems according to which sub-system""s algorithms can provide a more optimum suppression of the interference signals.
Applications with the most to gain from adding an ALOD system to an AAA system are those that, owing to platform size restrictions, cannot support an antenna system with many elements. A typical AAA system of interest might have only two to four antenna elements, for which the addition of an ALOD system can enable a substantial increase in jam-resistance.
The inventive system permits an adaptive antenna array with a given number of antenna elements to effectively suppress a larger number of interferers, and to adapt more quickly to a rapidly changing interference scenario. For an adaptive antenna array with a small number of antenna elements, the inventive system can provide added interference suppression capability with less increase in size, weight, power draw or cost than could be achieved by adding antenna elements to the adaptive antenna array.
The system is primarily designed for incorporation within systems using Direct Sequence Spread Spectrum signals, such as Global Positioning System (GPS) signals. The system is also appropriate for incorporation within systems using hybrid Direct Sequence and Frequency-Hopped Spread Spectrum signals, such as military antijam (AJ) radio signals. In a preferred embodiment the inventive system is incorporated within a conventional GPS receiver, which tends to be vulnerable to jamming by interference signals.